1. Field of the Invention
The present invention relates to data transmission and control systems for use in Integrated Services Digital Networks (ISDN), and more particularly to a data transmission and control system wherein D, Q, and M bits are locally and externally generated for inspection in X.25 information packets and transmitted to a location where they are separated out and used for user-selected control purposes.
2. Description of the Prior Art
Digital information is transmitted throughout an ISDN system in a serial manner with organizational bits inserted in the data stream for control functions. For example, in the telephone system a type of connection referred to as the Basic Rate Interface transmits data in 48 bit segments called frames containing two B channels (B1 and B2) and one D channel, each B channel functioning effectively at 64 kbps and the D channel at 16 kbps. The basic organization of the B and D channels is illustrated in simplified form in FIG. 1 showing the two B channel allocations separated by D channel allocations. The channels are sequences of dedicated bits in the serial bit stream with organizational bits (not shown) setting them apart from each other. A larger number of bits is allocated to the B channels to allow their use for high data rate requirements such as voice communication. The D channel of lower capacity is used primarily for set up and other intermittent data transmission purposes. Each B channel allocation contains 8 bits for data and each D channel allocation contains one data bit.
FIGS. 1B and 1C show the organization of the bits making up a D channel frame, the bits actually being distributed over a number of the frame sequences of FIG. 1A, the lines 10 indicating that each bit of the bit stream of FIGS. 1B and 1C is really one of a series of separated bits in the complete data stream of FIG. 1A. Information sent in the D channel is preceded by a standard Flag at position or field 12 followed by an Address field 14 and Control field 16. The Information field 18 is normally 128 bits maximum, followed by a Checksum field 20 and concluded with a Flag field 22. The bits included in the Information Field 18 are referred to as layer 3 and are further detailed in FIG. 1C. There is a General Format Identifier field (GFI) 24 including a Q bit 26 and D bit 28. This D bit is not to be confused with the use of the letter D for D channel as related to FIG. 1A where every D channel bit is designated by the letter D. The D bit 28 is a separate and distinct one of the D channel bits and reference to the D bit in the following refers to this D bit 28 specifically. Although it may be somewhat confusing, the terminology is standard in the art. The GFI field 24 is followed by a Logical Channel Number (LCN) field 30 identifying the Data Terminal Equipment relative to the ISDN network at each end of the communication path. The Packet Type field 32 keeps record of the number of packets sent and packets received (not shown), and contains the M bit 34. This is followed by the Data field 36.
The bits of FIGS. 1A, 1B, 1C are arranged according to a standard format defined under rules known as X.25 protocol. The X.25 protocol was designed to accommodate ISDN systems as shown in FIG. 2 containing asynchronous terminals such as 38 and 39. In order to connect the asynchronous terminals 38 and 39 to a synchronous packet switched network including nodes 48 and links 50, a device called a PAD 52 (packet assembler-disassembler) is required. Physically, the PAD 52 can be part of the network 42, or it can be a privately owned device as illustrated at 54. FIG. 2 shows asynchronous terminals 38 connected to a public network 40 and then to an ISDN packet switched data network 42, and terminal 39 connected through a PAD 54 and an X.25 access line 44 to the packet switched data network 42. The figure also shows a data terminal 46 with the built in capability of assembling and disassembling X.25 packets, and in such a case if an X.25 access line 44 is available, a direct connection can be made to the nodes 48 of the packet switching network 42.
The data links 44 connecting to the nodes 48 are called X.25 access links and are governed by the organizational structure defined in X.25 protocol. The PADs 52 and 54 perform the X.25 protocol functions that the asynchronous terminals 38 and 39 cannot manage by themselves. A set of rules called Recommendation X.29 describes the control messages which are sent between the PAD 52 and the terminal 38. The PAD 52 must distinguish between messages that are for terminal to PAD operation and those that contain user information to be sent through the nodes 48 and links 50 over the ISDN network 42. The Q bit 26 shown in FIG. 1C provides for communication between a PAD 52 and a terminal 38. When the Q bit is set at Q=1 the PAD is notified that the data in Data field 36 is meant for the PAD, and a Q=0 communicates that the data 36 is meant to be transmitted over the ISDN network 42.
In the case of the synchronous terminal 46, the Q bit is not needed, because the organization is all under direct user control. This represents a waste of bit space which could be used for some purpose if equipment to accomplish it were available. A further disadvantage with the prior art is that such a data terminal would be rather expensive and therefore limited to applications where considerable data communication is required.